#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
typedef double db;
int const xn=1005;
int n,s; db f[xn][xn];
int main()
{scanf("%d%d",&n,&s);for(int i=n;i>=0;i--)for(int j=s;j>=0;j--){if(i==n&&j==s)continue;db p0=1.0*i/n*j/s,p1=1.0*(n-i)/n*j/s,p2=1.0*i/n*(s-j)/s,p3=1.0*(n-i)/n*(s-j)/s;f[i][j]=p1*f[i+1][j]+p2*f[i][j+1]+p3*f[i+1][j+1]+1;f[i][j]=f[i][j]/(1-p0);}printf("%.6f\n",f[0][0]);return 0;
}

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
typedef double db;
int rd()
{int ret=0,f=1; char ch=getchar();while(ch<'0'||ch>'9'){if(ch=='-')f=0; ch=getchar();}while(ch>='0'&&ch<='9')ret=ret*10+ch-'0',ch=getchar();return f?ret:-ret;
}
int const xn=550;
int n,f[xn]; db p[20],A[xn],B[xn];
int main()
{int T=rd();while(T--){n=rd(); int k1=rd(),k2=rd(),k3=rd(),a=rd(),b=rd(),c=rd(),sum=k1+k2+k3;for(int i=0;i<=sum;i++)p[i]=0;p[0]=1.0/(k1*k2*k3);for(int i=1;i<=k1;i++)for(int j=1;j<=k2;j++)for(int k=1;k<=k3;k++)if(i!=a||j!=b||k!=c)p[i+j+k]+=p[0];for(int i=0;i<=n+sum;i++)A[i]=0,B[i]=0;for(int i=n;i>=0;i--){for(int k=3;k<=sum;k++)A[i]+=p[k]*A[i+k],B[i]+=p[k]*B[i+k];A[i]+=p[0]; B[i]+=1;}printf("%.10f\n",B[0]/(1-A[0]));}return 0;
}

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#define eps 1e-10
using namespace std;
typedef double db;
int rd()
{int ret=0,f=1; char ch=getchar();while(ch<'0'||ch>'9'){if(ch=='-')f=0; ch=getchar();}while(ch>='0'&&ch<='9')ret=ret*10+ch-'0',ch=getchar();return f?ret:-ret;
}
int const xn=10005;
int n,hd[xn],ct,to[xn<<1],nxt[xn<<1],d[xn];
db K[xn],E[xn],P[xn],A[xn],B[xn],C[xn];
void add(int x,int y){to[++ct]=y; nxt[ct]=hd[x]; hd[x]=ct;}
void dfs(int x,int fa)
{db as=0,bs=0,cs=0;for(int i=hd[x],u;i;i=nxt[i]){if((u=to[i])==fa)continue;dfs(u,x);as+=A[u]; bs+=B[u]; cs+=C[u];}A[x]=(d[x]*K[x]+P[x]*as)/(d[x]-P[x]*bs);B[x]=P[x]/(d[x]-P[x]*bs);C[x]=(P[x]*cs+d[x]*P[x])/(d[x]-P[x]*bs);
}
int main()
{int T=rd(),cnt=0;while(T--){cnt++; n=rd(); ct=0; memset(hd,0,sizeof hd); memset(d,0,sizeof d);for(int i=1,x,y;i<n;i++)x=rd(),y=rd(),add(x,y),add(y,x),d[x]++,d[y]++;for(int i=1;i<=n;i++)scanf("%lf%lf",&K[i],&E[i]),K[i]/=100,E[i]/=100,P[i]=1-K[i]-E[i];for(int i=1;i<=n;i++)A[i]=B[i]=C[i]=0;dfs(1,0);printf("Case %d: ",cnt);if(fabs(A[1]-1)<eps)puts("impossible");else printf("%.8f\n",C[1]/(1-A[1]));}return 0;
}